With the growing concern concerning the disposal of articles containing heavy metals it is highly desirable to develop inorganic pigments which are heavy metal free. Historically, inorganic pigments for plastic and paint applications in the yellow to red range have included CdS-CdSe, lead molybdate-lead antimonate, and iron oxide pigment families. The former two contain elements which are classified as "heavy metals" and their use is being stringently regulated. The iron oxide pigments are somewhat acid soluble and therefore cannot be used in all applications.
The use of praseodymium doped zirconium silicate (zircon) crystals as a pigment for use in ceramic glazes was disclosed by C. A. Seabright in U.S. Pat. No. 2,992,123. Since that time there have been numerous patents issued for praseodymium doped zircon for ceramic applications and it is now manufactured worldwide for ceramic applications. The application of this pigment to non-ceramic applications such as in plastics and paints has previously not been disclosed.
The importance of the crystal size of a pigment and the particle size distribution of the pigment powder to the performance of the pigment is well known in the industry. The physics of the optics of small particles can be found, for example, in the book "Absorption and Scattering of Light by Small Particles", by C. F. Bohren and D. R. Hufman, John Wiley and Sons, 1983. In U.S. Pat. No. 4,752,341, S. L. Rock discloses the importance of the particle size of the zeolite when used as a pigment with titanium dioxide in paper. In U.S. Pat. No. 4,767,464, Strauch et al. disclose the importance of the particle size distribution of carbonates used as fillers or pigments in dyes, paints, paper or plastics.
None of the foregoing technology describes stable, heavy metal free zircon pigments having a uniform distribution of particle size, allowing the pigment to be used for coloring plastics and paints.